Remove an unused enhanced bookmark histogram

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 741dded356f3f49f567de14b1789ca972b356639..a7fd81ad3c7ed865934a4ad1164217fd474f885c 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -8993,6 +8993,9 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 
 <histogram name="EnhancedBookmarks.ViewMode" enum="EnhancedBookmarkViewMode">
   <owner>ianwen@chromium.org</owner>
+  <obsolete>
+    Removed 9/2015 after list view became the only view mode.
+  </obsolete>
   <summary>
     Record which Enhanced Bookmark UI mode users are in. This will be recorded
     everytime the user opens the bookmark manager UI.
@@ -55103,6 +55106,9 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 </enum>
 
 <enum name="EnhancedBookmarkViewMode" type="int">
+  <obsolete>
+    Deprecated 9/2015.
+  </obsolete>
   <int value="0" label="Default view mode"/>
   <int value="1" label="List view mode"/>
   <int value="2" label="Grid view mode"/>